1. Field of the Invention
The present invention relates to a system for detecting certain classes of errors in data communicated on a data bus in accordance with a certain protocol defining signal configuration and word format.
2. Description of the Prior Art
In many digital data networks, information is communicated between subsystems or terminals via a data bus that is time shared. An example of such a time division multiplexed data bus architecture is that utilized for the intercommunication of information between the electronics subsystems in a military aircraft.
To ensure proper communication of data on such a bus, a certain bus protocol is established. This defines the signal shape and data word format of all information that is to be transmitted via the bus. In the case of military aircraft electronics systems the data bus protocol is established by a certain military standard MIL-STD-15553A and B. While certain reference will be made herein to the data encoding and word format configuration used by this standard, the invention is not so limited, but can be used in connection with other bus protocols and other data bus applications, such as computer networks with their associated protocols.
FIGS. 1 and 2 illustrate the signal configuration (i.e., data encoding) and word format respectively for the protocol used in MIL-STD-1553, and is exemplary of other data bus protocols. The data signal is transmitted on the bus in Manchester II biphase L coding, in which the data signal alternates between positive (+) and negative (-) voltage level. In each bit time there is a mid-bit transition in voltage level from postive to negative (which represents a binary "1") or from negative to positive (which represents a binary "0"). The bit time duration is fixed. In MIL-STD-1553 the transition bit rate is 1.0 megabit per second, so that each bit time is one microsecond in duration.
Data is transmitted in the form of a word having twenty bit times (FIG. 2). The first three bit times are used to transmit a sync signal. This differs from the data bit signal in that the sync time duration is equal to three bit times, with a voltage transition occurring only at the center of the second bit time. There are two sync configurations, which are used to distinguish data words from command or status words. For a data word, the transmitted signal is of negative polarity for the first one and one-half bit times, has a negative to positive transition at the center of the second time, and thereafter is positive for one and one-half bit times. For a command or status word, the sync pulse is positive for the first one and one-half bit times, has a positive to negative transition at the center of the second bit time, and thereafter is negative for one and one-half bit times.
During the next bit times 4 through 19, sixteen data bits can be transmitted, each having the data encoding characteristic shown in FIG. 1. For a data word, the bits can have arbitrary significance. For a command or status word, the bits designate certain specified information such as remote terminal address, data word count etc., (as shown in FIG. 2). During the final (20th) bit time, a parity bit is transmitted. Blocks of up to thirty-three such words (typically including the command word), one immediately following another, may be sent on the bus. The presence of the sync pulse designates the beginning of the following word.
Various errors can occur in the transmission of data on the bus. For example, degradation of the signal waveshape may occur so that the transitions between positive and negative levels become distorted and well-defined, easily detected transitions do not properly occur at the mid-bit time. Circuitry malfunction may cause transmission of too many or too few bits per word. In another error mode, no transition at all may occur during a certain bit time, so that it cannot be determined whether the transmission of a binary "1" or "0" was intended.
An object of the present invention is to provide a system for detecting and analyzing certain classes of errors which might occur on a bus handling data in accordance with a certain bus protocol. A further objective is to provide such a system for analyzing data encoded in a biphase format. The system is capable of determining the presence of a biphase transition, and of determining whether such transition is of sufficient sharpness (i.e., not distorted beyond acceptable recognition) and occurs within acceptable time tolerances with respect to the actual center of the bit time.
Another object of the present invention is to provide an error detecting system which continuously looks for the presence of a sync signal, even during those times when data bit transmission is expected. This capability enables the system to determine a class of error which might occur if a word is transmitted having too few bits, immediately followed by another word or by the bus going inactive or "dead". In such instance, the beginning of the subsequent word would immediately be sensed by the detected presence of the sync signal. The presence of too few bits during the preceding word concomitantly would be determined, so that that word can be flagged as an error.